Charge-Transfer Complex Hybrid Films Consisting of Proton Conductive Donor Molecules for High Temperature PEM

Stable PEFC operation more than 100 ˚C provides various merits such as high CO tolerance, improvement of fuel efficiency and gives a significant reduction in the cost of PEFC system and increases in power yield. In order to realize stable HT-PEFC, development of new materials for HT-PEFC are strongly required. Polymer electrolyte membranes (PEMs) also have to be developed the new alternative materials which have the stable performance in the long-term operation at high temperature condition. Especially, the control of the proton conductivity at high temperature is very important because relative humidity at high temperature is lower than that in standard operation condition.

There are several approaches to develop an alternative PEM for high temperature operation. Straightforward approach is modification of molecular design of HT-PEM by organic synthesis. Other practical approach is to incorporate PEMs with proton transporters such as ionic liquids, polyacids. Polymer processing to develop the alternative PEMs has been also reported. Non-woven PEM by electrospinning method is one of the attractive materials which we can control the functions and the morphology of the polymer fibers. These approaches are promising methods to realize the alternative HT-PEMs. However, each approach still has drawback such as complicated synthesis process, leaching the proton conductive additives, limitation of the operation condition and so on. Especially, development of simple and easy processing to prepare the new HT-PEM with the high performance has been not realized yet.

To solve this problem on the processing of HT-PEM, we have developed the simple film preparation method to prepare the functional HT-PEM by using supramolecular chemistry1,2. As supramolecular interaction, charge-transfer (CT) complex was used in PEM. Preparation of CT complex hybrid film is very simple; mixing electron-donating molecules and the electron-accepting polymer, and evaporating the solvent from the mixture. CT complex formed by polymer and the additives in PEM could work as a physical molecular binder between polymer chains. The property of the CT complex hybrid films such as mechanical strength was improved by CT complex. The other advantage of CT complex hybrid films is that control of the film properties is feasible by changing the concentration of the CT complex and the added donor species.

In this study, we synthesized a donor molecule with sulfonic acid (SDPN) and the derivatives and prepared the CT complex hybrid films with electron-accepting sulfonated polyimide (SPI). We evaluated the various properties such as proton conductivity.

The obtained films showed brown color which indicated CT complex formation. From the result of the visible spectra, absorbance of CT complex between SPI and SDPN was confirmed. The obtained CT complex hybrid films were used for proton conductivity measurement to the through-plane direction of the films. At high temperature condition (100-120 ˚C), the CT complex hybrid films showed about 100 mS/cm at 100 RH%. These values are higher than Nafion 212. We also evaluated humidity effect to the CT complex hybrid films at high temperature. The proton conductivity of the CT films decreased with the decrease of humidity at 120 ˚C. However, the proton conductivity of the CT films was higher than Nafion 212 at all humidity.

1. Ryohei Watari, Masamichi Nishihara, Hiroo Tajiri, Hideyuki Otsuka, Atsushi Takahara,, Polymer Journal, 45, 839–844, 2013

2. Liana Christiani, Sandra Hilaire, Kazunari Sasaki, Masamichi Nishihara, Journal of Polymer Science, A, 52, 2991–2997, 2014